SCUBA (Self-Contained Underwater Breathing Apparatus) operations typically require that rate of change in depth be limited to typically no more than 60 feet per minute. This maximum rate of change in depth gives the divers' bodies enough time to assimilate to the pressure change. Exceeding this rate can cause damage to inner ears and more serious pressure-related medical difficulties such as embolism and the bends. During normal scuba diving without any supplemental propulsion, it is fairly easy to stay below this maximum rate of change in depth. When scuba divers travel faster through the water using propulsion devices, it become much easier to exceed the maximum rate of change in depth.
Propelled underwater movement has many advantages over normal underwater human powered travel. These supplemental propulsion devices provide faster underwater movement and allow a diver to cover greater areas and travel farther when underwater time is limited. Almost all underwater activities are limited by the supply of air available to the diver and the stored power available for the propulsion mechanism that is typically an electric motor. Examples of propelled underwater movement include a powered water scooter that pulls the operator underwater and more sophisticated submersibles that allow one or more divers ride inside or on a propelled craft. These enhanced underwater speed capabilities are useful in military operations (swimmer delivery vehicles) as well as recreational, sport, commercial, and scientific driving activities using SCUBA equipment.